Project Summary Abstract A major limitation of endocrine therapies targeting estrogen receptor ? (ER?) is the development of acquired resistance to therapies such as tamoxifen in initially responsive patients. Altered microRNA expression plays a role in endocrine resistance; however, little is known about what regulates microRNA in breast cancer. This exploratory study addresses PA- 16-177 ?Role of RNA modifications in cancer biology (R21) Question 1: ?What are the cancer- associated alterations in readers, writers, and erasers of reversible RNA modifications?? Here, we will address the role of the RNA methylation reader Heterogeneous Nuclear Ribonucleoprotein A2/B1 (HNRNPA2B1) in tamoxifen-resistant breast cancer. The rationale for our proposed experiments is that increased N(6)-methyladenosine (m6A) in primary-microRNAs (pri-miRNAs) was associated with increased levels of the corresponding mature miRNA in a triple negative breast cancer cell line. HNRNPA2B1 is a reader of the m6A mark in pri-miRNAs and interacts directly with DGCR8 (a component of the DROSHA complex in the nucleus) to promote processing of pri-miRNAs to precursor-miRNAs (pre-miRNAs). We present preliminary data showing that HNRNPA2B1 is increased in tamoxifen-resistant, ER?+ LCC9 cells derived from tamoxifen-sensitive, ER?+ MCF-7 breast cancer cells. Using online data, we found that high HNRNPA2B1 transcript levels in primary breast tumors are associated with reduced overall survival. A new report showed increased HNRNPA2B1 in breast tumor specimens and cell lines. The proposed experimental plan addresses the hypothesis that HNRNPA2B1 overexpression stabilizes pri-miRNAs that promote endocrine resistance.